This document introduces cross-platform mobile development. It discusses the history of smartphones from 1973 to today. It notes the rise of Android, iOS, and other platforms. Cross-platform development allows code reuse for developers and reduced costs for companies using web technologies like HTML, CSS, and JavaScript. While cross-platform apps have limits in performance and graphics compared to native apps, cross-platform tools like PhoneGap, Titanium, and Unity3D allow developing for multiple platforms from a single codebase. The document considers whether cross-platform or native development is best.